As WS states, IF you are fully trained in your home country and IF a PD in the US is willing consider that adequate then you can come to the US and do just a fellowship. If you do 3 years of fellowship, you should be able to apply for a license in the US. However:
1. You may NOT be eligible for board certification. In IM, you must be core IM certified to become subspecialty certified, so you would not be eligible for board certification. This might seriously affect your ability to be employed (although you could open a private practice). You would likely not be credentialable at most hospitals. Other fields might be more lenient.
2. This all goes away in 2016. Starting then, all prior training must be ACGME. Only international ACGME-I training would be considered acceptable (and still does not qualify for the board certification issue above)